#1
OK, I found out about timing a couple of weeks ago and I've been baffled since.
I watched a couple concert videos and I see guitarist DOWN picking on the 8th notes but when I learned about timing, tutorial videos said that you have to alternate pick on the 8th notes

1(down) and(down)
or
1(down) and(up)?

Does it depend on different types of music?
e.g. Metal: constant down picking
#2
Generally, alternate picking is the preferred way of playing. It is just more economical to play that way. However, strict downpicking has a sound of its own. You can't really play Metallica and make it sound the same without downpicking, for example. It is a style thing.
#3
There's not really a right way or a wrong way; alternate picking produces one sound, downpicking produces a different sound. It just depends on what you want.
#4
Depends on your goal. Like already mentioned, all down picking creates a heavier sound. However, just because it's an 8th note doesn't really determine the speed either. If you were playing 8th notes at 40 bpm it would make sense to down pick everything. If you were playing 8th notes at 200 bpm then it would make sense to alternate pick everything.

When teaching my students how to count and play 8th note rhythms I do have them alternate pick to get used to a steady back and forth motion for smooth sounding rhythm. When we move to 16th notes the picking pace is doubled, so if a measure contains just 8th notes they would all be down picked, and the 16th notes would be alternate picked.
#5
Can't I just down pick the notes that are on the beat and right between the beats and just up pick everything of beat.

e.g. 1(down) e(up) and(down) a(up)
#6
Quote by MadFisto
Can't I just down pick the notes that are on the beat and right between the beats and just up pick everything of beat.

e.g. 1(down) e(up) and(down) a(up)


Yes. That's exactly what I was implying when it comes to picking 16th note patterns. So you can definitely down stroke where you wrote the down strokes.

Honestly, you can down pick 16th notes too. Even 32nd notes. It really depends on what the BPM is. Down picking 16th notes at 60 bpm is very doable.

I think the main thing to keep in mind is keeping your rhythm nice and fluent. If it's in time and on the beat, you're good!